Heat resistant, anti-corrosive alloys for high temperature service

ABSTRACT

The present invention relates to a heat resistant anticorrosive alloy characterized by good resistance to corrosion at high temperatures and high strength at high temperatures which is suitable as material for re-combustion-type emission gas purifiers such as thermal reactors, after-burners, more specifically to a metal alloy substantially composed of ferrite phase, its major components being: CARBON...BELOW 0.15% AT LEAST ONE OF NIOBIUM AND Tantalum...0.05-3.0% chrome...12-28% Titanium...0.05-1.0% molybdenum...0.2-3.0% Silicon...below 2.0% aluminum...0.3-6.0% vanadium...0.1-3.0% boron...0.0001-0.0050% zirconium...0.01-1.0% iron...balance AND, IF NECESSARY WITH ADEQUATE ADDITIONS OF THE FOLLOWING ELEMENTS: One or more than two from among rare earth elements such as yttrium, cerium lanthanum, calcium . . . 0.01-1.5%; TUNGSTEN . . . 0.2-3.0 PERCENT; BERYLLIUM . . . 0.01-2.0%.

ite te 11 1 Niimi et a1.

[111 3,852,063 Dec. 3, 1974 HEAT RESISTANT, ANTI-CORROSIVE ALLOYS FORHIGH TEMPERATURE SERVICE [75] Inventors: ltaru Niimi, Nagoya; YasuhisaKaneko, Toyota; Masamitu Noguchi, Toyota; Tsuneo Uchida, Toyota; YouheiKatori, Toyota, all of Japan [73] Assignee: Toyota Jidosha KogyoKabushiki Kaisha, Aichi-ken, Japan 221 Filed: Oct. 4, 1972 21 Appl. No.:294,981

Primary Examiner-Hyland Bizot Attorney, Agent, or Firm-Stevens, Davis,Miller & Mosher [57] ABSTRACT The present invention relates to a heatresistant anticorrosive alloy characterized by good resistance tocorrosion at high temperatures and high strength at high temperatureswhich is suitable as material for recombustion-type emission gaspurifiers such as thermal reactors, after-burners, more specifically toa metal alloy substantially composed of ferrite phase, its majorcomponents being:

carbon... below 0.15% at least one of niobium and Tan1alumn.O.053.O/1

Titanium...0.051i0/( Silicon...below 2.071

chrome...l2-28% molybdenum..1O.2-3.07: aluminum...0.3-6t0%vanadium...0.13.0% b0ron...0.000 l-0.0050% zirconium...0.0 l -l .07:iron...balance and, if necessary with adequate additions of thefollowing elements:

One or more than two from among rare earth elements such as yttrium,cerium lanthanum, calcium ...0.01-1.5%;

tungsten 0.2-3.0 percent; beryllium 0.0l-2.0%.

4 Claims, 5 Drawing Figures (ti)(b)(c)(d)(e)(f1(g)(h)(j)(q)(t)(w)(,i) AB C D E F G H PATENTEL 3 74 I saw 10F 4 A 68 x oem:

L IL FL PATENTELnac 31974 SHEET 2 OF 4 A :mm x 0 00M:

I w m 0 Q m E31:31:EIBCIBGI IQIB N O m O v6 PATENTh 553 31974 SHEET 30F4 26 x o oom:

I o h. m a o m 2: 1:3::2132132 132: c:

PATENTEL DEC 31974 FIG.4

BACKGROUND OF THE INVENTION Both abroad and at home, the control of autoemission gases has come to be enforced with increasing stringency yearafter year. To accomplish such control effectively, it is necessary toequip the conventional internal combustion gasoline engine with newdevices for emission gas purification such as thermal reactors, afterburners, catalyst muffers or E.G.R. (exhaust gas recirculation) or totry engine modifications.

In the re-combustion type emission gas purifier which has to work at amaximum temperature of 1,200C, its component members, which are exposedto high temperature combustion gases, must be able to satisfactorilywithstand the oxidation due to CO H and residual 0 contained in thesegases and the high temperature corrosion due to PbO, S, S0 or Pcontained in them. Besides, to assure ample durability of the apparatusitself, its members have to be strong enough at high temperatures.

As materials for thermal reactors and after burners, Fe-Cr-Al alloysconventionally employed in furnace heaters or various heat-resistantstainless alloys are conceivable. The conventional Fe-Cr-Al alloys excelin high temperature corrosion resistance, but they have the drawbacksthat they are liable to become brittle in high temperature servicethrough coarsening of crystals and they lack high temperature strength.Meanwhile, austenitic stainless alloys and nickel base heat-resistantalloys, characterized by excellence high temperature strength, lack inhigh temperature corrosion resistance and accordingly they need surfacetreatment; or they are so expensive that they are found unfit for massproduction.

The present inventors, setting an eye on the excellence of Fe-Cr-Alalloys in their high temperature corrosion resistance, investigated howto improve these alloys in the coarsening of their crystals andconsequently from their investigation have successfully perfected thepresent invention of an alloy with remarkably better characteristics ofhigh temperature corrosion resistance than that of the conventionalaustenitic stainless alloy or nickel base heat-resistant alloy, andbetter characteristics of high temperature strength, crystal coarseningand moldability than those of the conventional Fe-Cr-Al alloys.

SUMMARY OF THE INVENTION The object of the present invention is toprovide an alloy suitable as material for component members of are-combustion-type emission gas purifier which is characterized byexcellence in high temperature corrosion resistance, high temperaturestrength, crystal coarsening characteristics and moldability, the majoralloying elements to constitute this alloy to attain this object bemg:

carbon below 0.l5% zirconium 0.0l-l.0% chrome 12-2 8% iron balancemolybdenum 0.2-3.0% at least one of niobiam and tantalum ODS-3.0% I

titanium 0.05- l .0%

silicon below 2.0%

aluminum 03-60% vanadium 0. l 3 0% boron 0.000l-0.0050% and, ifnecessary, the following elements being adequately added:

One or more than two from among rare earth elements such as yttrium,cerium, lunthanum, calcitungsten 0.2-3.0 percent; beryllium 0.- 01-20%.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing theresults of comparison in anti-oxidization characteristics between theinvented alloy and various conventional heat-resistant anticorrosivealloys,

FIG. 2 is a diagram showing the results of comparison in corrosionresistance in a hot atmosphere of lead oxide between the invented alloyand various conventional heat-resistant anti-corrosive alloys,

FIG. 3 is a diagram showing the results of comparison in corrosionresistance in a hot atmosphere of sulphur between the invented alloy andvarious conventional heat-resistant anti-corrosive alloys,

FIG. 4 is a micrograph showing an intercrystalline corrosion recognizedin an austenitic alloy (SUS 41) which has been etched by hot sulphur,

FIG. 5 schematically illustrates a specimen used in a Charpy test of theinvented alloy.

DETAILED ACCOUNT OF THE INVENTION The present invention relates to analloy characterized by excellence in high temperature oxidationresistance, high temperature corrosion resistance in an at mosphere oflead oxide or sulphur and high temperature strength which can be appliedas material for heatresistant members in various re-combustion-typeemission gas purifiers and for vessels in a catalyst muffler, for engineparts exposed to hot emission gases, for components of gasturbinecombustion chambers and for furnace heaters.

The invention alloy is substantially constituted by a ferrite phase, itschemical composition being broadly as follows carbon below 0.15% siliconbelow 2.0% chrome l228% aluminum 03-60% molybdenum 0.23.0% vanadium0.l-3.0%

at least one of niobium and tantalum ODS-3.0% boron 0.000l0.0050%titanium ODS-1.0% zirconium 0.01-1 .0%.

iron balance O.23.0 percent; beryllium markedly reduced by addition ofSi or excessive addition of B. Addition of Y greatly improves the impactwithstanding characteristics. Addition of Cr, A1, Y is effective forimproving the anti-oxidation, anti-lead oxide, and anti-sulphurcharacteristics, but excessive addition of B is harmful.

TABLE 1 Effects of various additives in the invented alloy on variouscharacteristics.

TABLE 1.EFFECTS ()F YARN) US ADIHIIVES IN THE INYENTEI) ALLOY ()N YA1110118 ("11A RAU'II", 111S'l1( 1 Elements Characteristics (1 Si Cr A1Mo Tu N1) 'li Zr ll 111 Y High temperature strength... O A O Q GD m.((1) [1 3 (1 1 A Crystal coarsening E1 E1 13 G (O [.1 D O O 1Roomtemperatureelongation A X A A El 151 A [.1 [J ill 11 Impact strengthA A A A O A U [3 O [L1 1 1 rtntioxidotion A O to (g) 13 :1 u U A L1 {A uAnti-load oxide" A A to 1:1 0 A o u u {1.1 Anti-snlphur A O 13 111 [1 C15 L] 1.1 (J

N0'rE. 1\Iark means that the relevant characteristic has been vastlyimproved. O means that the ellt-et, though nnl ennspicuous, is positive.[1 means that the l-tluct is insignificant or practically Zero. A meansthat. the elleet 15 rather negative. X means that the effect is markedlynegative.

TABLE 2 Chemical compositions of alloys tested.

Chemical elements C Si Mn P S Cr Al Ni Others Remarks Specimen codesa(conventiona1 alloy 0.06 0.57 0.46 16.51 (SUS24) b( do. 0.05 0.82 1.0918.29 9.20 (SUS27) c( do. 0.07 0.72 1.40 22.40 12.85 (SUS41) d( do. 0.061.05 1.68 24.74 19.89 (SUS42) e( do. 0.07 0.67 0.95 19.90 32.45(1nco11oy800) f( do. 0.05 0.56 0.62 21.39 45.10 Fe (Hastelloy X) g( do.0.02 0.46 0.53 15.70 Bal 8.48 (1nconc1l600) Fe h( do. 0.05 0.25 0.9404959 19.80 8:11 0.63 (Nimonick 75) Ti Zr i( do. 0.06 0.44 0.82 0.0320.008 19.65 3.72 0.06 0.34 0.11 (FCH j(tentative 0.01 0.08 0.l 0.0070.012 16.23 1.92 0.08 (Conventional alloy) alloy) k( do. 0.01 0.05 do.0.006 .006 16.60 1.77 0.01 0.16

Zr 1( do. 0.01 0.05 do. 0.005 0.012 15.60 2.48 0.03 0.11

. Ti m( do. 0.01 0.06 do. 0.007 0.011 16.08 2.29 0.04 0.93 n(tentativealloy) 0.01 1.36 do. 0.005 0.015 16.32 2.03 0.1

B o( do. 0.01 0.71 do. 0.005 0.011 16.24 2.55 do. 0.0014

Ti Y p( do. 0.01 1.01 do. 0.006 0.009 16.23 2.28 do. 0.20 0.18 q( do.0.01 0.06 0.1 0.007 0.012 15.47 4.60 g0] (conventional Ti alloy) r( do.0.01 0.02 do. 0.003 0.012 16.49 4.32 do. 0.14

V s( do. )0.01 0.03 do. 0.003 0.012 16.53 4.17 do. 1.29 t( do. 0.01 0.09do. 0.006 0.015 20.86 1.91 do. (conventional) Mo alloy) u( do. 0.01 0.02do. 0.007 0.012 20.56 2.08 do. 1.00

Nb v( do. 0.01 0.04 do. 0.004 0.016 20.11 1.74 do. 1.44 w( do. 0.01 1.30do. 0.005 0.014 21.10 4.22 do. (conventional B alloy) x( do. 0.01 0.66do. 0004 0.012 20.81 4.27 do. 0.12

B y( do. 0.01 0.06 do. 0.005 0.014 21.09 4.12 do. 0.37

Ti B z( do. 0.01 0.09 do. 0.003 0.011 21.13 4.31 do. 0.05 0.70

TABLE 2 Continued Chemical compositions of alloys tested.

Chemical elements C S| Mn P S Cr Al Ni Others Remarks Specimen codesA(lnvented Mo V Ti Zr Nb+Ta Y alloy) 0.06 0.38 0.55 0.014 0.011 17.332.82 0.24 1.00 0.57 0.41 0.17 0.12 0.03

Mo V Ti Zr Nb-l-Ta Y B( do. 0.05 0.43 0.68 0.009 0.006 16.75 2.76 0.191.06 0.52 0.36 0.16 0.52 0.04

Mo V Ti Zr Nb+Ta Y C( do. 0.07 0.36 0.63 0.013 0.008 16.98 3.13 0.231.00 0.58 0.39 0.18 0.12 0.37

Mo V Ti Zr Nb+Ta B D( do. 0.06 0.29 0.62 0.014 0.014 20.08 3.00 0.200.50 0.56 0.37 0.16 0.49 0.0009

Mo V Ti Zr Nb-l-Ta B E( do. 0.06 0.31 0.62 0.014 0.014 20.21 2.90 0.201.00 0.56 0.41 0.17 0.50 0.001

' Mo V Ti Zr Nb+Ta B F( do. 0.06 0.29 0.66 0.014 0.018 19.99 2.72 0.260.51 1.10 0.41 0.14 0.48 0.0009

- Mo V Ti Zr Nb-l-Ta B G( do. 0.06 0.27 0.62 0.014 0.016 19.99 3.05 0.291.11 1.08 0.40 0.16 0.51 0.0007

. Mo V, Ti Zr Nb+Ta B Be l-1( do. 0.06 0.32 0.64 0.013 0.014 3.07 0.200.53 0.54 0.38 0.16 0.47 0.0006 0.07

In the following, the features of the invented alloy will be describedin examples of testing.

Table 2 is a list of chemical compositions of speci- 1n FIGS. l-3 andTable 3, the characteristics of the.

invented alloy and various specimens are compared.

'FIG. 1 shows the etched depths of various alloys as measured afterbeing submitted to 200 hours of oxida-' tion in an alumina boat in anatmospheric furnace pr'eliminarily heated to 1,200C; these .depths'represent the high temperature oxidation characteristic of these alloys.It is evident from FIG. 1 that some of the commercial heat-resistantstainless alloys, for instance, the specimens (g) (h) exhibitconsiderably good antioxidation characteristic, but even compared withthem, the invented alloy as well as the known Fe-Cr-Al alloys is farsuperior in this characteristic. It should be noted, however, that inthe case of Fe-Cr-Al alloys the contents of Cr and Al affect theanti-oxidation characteris tic to a certain extent, but the inventedalloys (A)-(C) and the tentative alloys (K) and (P), which contain Y andare added with relatively little Cr and Al, exhibit aremarkableexcellence in his characteristic which is a striking featurewhen it is recalled that in the case of Fe-Cr-Al alloys, reducedcontents of Cr and Al vastly improve the moldability.

FIG. 2 shows the etched depths of various alloys which represent theircorrosion resistance in a hot atmosphere of P O, as measured after thesealloys had been washed, degreased, coated with a slurry mixture of Pb 0powder and waterglass solution(water: water glass 4:1 in volume ratio of1:3; placed in an alumina boat; and then held for hours in a furnacepreliminarily heated to 1200C.

In nearly the same way as in the results of antioxidation test, allFe-Cr-Al alloys including the known ones and the invented one aresuperior in corrosion resistance in a hot atmosphere of Pb O to any ofthe conventional commercial heat-resistant stainless alloys.

Similarly, FIG. 3 shows the etched depths of the specimens whichrepresent their corrosion resistance in a hot atmosphere of sulphur, asmeasured after the specimens were coated with a slurry'mixture of powdersulphur and waterglass solution (water: waterglass 1:1) in volume ratioof 1:3; placed in an alumina boat; and held for 6 hours in a furnacepreliminarily heated to 1,200C.

These results, just as those of oxidation tests and hot Pb O corrosiontests, point to the superiority in corrosion resistance in a hotatmosphere of sulphur of Fe- Cr-Al alloys including the known ones andthe invented one to any of the conventional commercial stainless alloysor heat-resistant alloys; and these Fe-Cr-Al alloys are found free fromany intercrystalline corrosion in FIG. 4 which is recognized in asulphur-etched austenitic alloy or nickel base alloy.

Table 3 summarizes the characteristics of the conventional Fe-Cr-Alalloys and the invented Fe-Cr-Al alloy, such as strength at roomtemperature and at high temperature; elongation at room temperature;impact strength; and crystal coarsening. As seen from this Table, theinvented alloys (A)(H) are remarkably better in high temperaturestrength characteristic than any Fe-Cr-Al alloy in the prior art;particularly the alloy (H) is about two times superior to any one of theconventional (j), (q), (t), (w) and (i).

This is probably due to the great effects of B, Be, Mo, Ta, Nb and V, astestified by the performances of the tentative alloys (0), (r), (s),(u), (v), (x), (y) and (z); to be more specific they are mainly theeffect of solid solution by invasion in the case of B, that of solidsolution by substitution in the case of the other elements; and partlythat of precipitation of carbides.

Concerning the room temperature elongation characteristic, the contentsof Cr, Al and particularly that of A1 are supposed to made greatcontributions; in this characteristic, the invented alloys are generallysuperior to the known (i), (j), (q), (t) and (w). This is, as suggestedby the performances of the tentative alloys (r), (n), (o), (s),presumably due to the addition of Ta being small and those of B, Si andV being well controlled.

The impact strength mentioned in the Table is ex pressed by the energyabsorption needed to break a specimen of a special shape indicated inFIG. 5, using a kg-m small size Charpy impact tester, as divided by theoriginal cross-sectional area of the specimen.

The contributions of Cr, and Al particularly the latter to this impactstrength are supposedly immense; the invented alloys are in generalsuperior in this strength to any of the known (i), (j), (g), (t) and (w)which contain the same amounts of Cr and Al as the invented alloys. Assuggested by the performances of tentative alloys, this seems to be dueto additions of Y and Mo and restrictions of Si, B and V contents. It isnoteworthy, as suggested by the performances of the tentative alloy (k)and the invented ones (A) (B) and (C), that the alloys added with Y aremarkedly improved in impact strength.

The crystal coarsening characteristic in Table 3 is expressed in termsof temperatures at which the ferrite crystal size becomes less than 1when each alloy specimen has been held for 25 hours in the range of800-l,200C at intervals of 50C and then observed for coarsening ofcrystals as the results of heating. An increase in the contents of Crand Al is apt to be accompanied by a slight rise in the coarseningtempera- TABLE 3 Comparison of various properties among differentFe-Cr-Al alloys (known, tentative and invented) Room temperatureelongation High temperature tensile strength Kg/mm lmpact StrengthSpecimen Kglmlcol codes Crystal coarsening temperature (C) service as athermal reactor, the invented alloy receives less thermal strain due toheating and cooling and accordingly suffers less thermal fatigue.

Carbon as a solid solution constitutes a part of the alloy andstrengthens it. As combined with elements such as Mo, V, Nb, it formscarbides, which contribute to the strengthening of the alloy, but itscontent should be limited to less than 0.15 percent, because too muchcarbon is likely to cause intercrystalline and intracrystallineprecipitations of carbides (or nitrides) heated in the process, afterpermeating the carbon into the texture in the form of a solid solution;and thus, corrosion resistance is reduced through lack of Cr in thevicinity of the precipitates and with this, the moldability too becomespoor.

Si is effective for improving anti-oxidation, antisulphur and anti-PbOcharacteristics, but Si not as effective as Cr or Al. Meanwhile Si islikely to cause heavy deterioration in the moldability at roomtemperature and impact strength if used in large quantities. Thus, it isadvisable to limit its content to less than 2 percent.

Cr as a solid solution in the texture is indispensable for giving heatresistance and anti-oxidation characteristics to the alloy and securingits ferrite structure.

Thus, its content should be more than 12 percent, but should not exceed28 percent, because its excess deteriorates the moldability orweldability and is likely to cause 475C brittleness and cr-phaseprecipitation.

The favorable effect of Al on anti-oxidation, antisulphur and anti-PbOcharacteristics is prominent and its effect on high temperaturestrength, though insignificant, is also recognized. its effects,however, are not so great when its content is to little. Thus, 0.3percent is the lower content limit on the contrary if 6 percent is(i)known (j)tentative (k) do.

(A)invehted 1200 up 1200 up 1200 up 1200 up 7 1200 up 1200 up 1200 up1200 up 1200 up 1200 up 1200 up exceeded, the moldability andweldability are adversely affected to a great degree.

Mo, partly as a solid solution in the texture and partly as a carbide,is found useful for strengthening the alloy and preventing thecoarsening of its crystals.

Too much Mo, however, results in deterioration of the anti-oxidationcharacteristic and corrosion resistance at high'temperatures. Theappropriate content of Mo would, be 0.240 percent. W exhibits a similareffect as Mo, so a partial or whole content of Mo may be substituted byW.

V, similarly to M0, serves to strengthen the alloy partly through, itseffect as a solid solution in the texture and partly through its effectas a carbide. Too much V,

however, willresult in reduction of its elongation at room temperature,its impact strength and its corrosion resistance at high temperaturessuch as its antioxidation characteristic. Thus, the recommendablecontent of V should be 0.12.0 percent.

Ta, when added to Fe-Cr-Al alloys, behaves uniquely; thereby a smallamount of Ta can improve the high temperature strength, thecrystal-coarsening characteristic and the room temperature elongation;but its excessive addition will heavily reduce the impact strength ofthe alloy. Thus, the right addition of Ta should be 0.05-3.0 percent.The effect of Nb is nearly the same as that of Ta. Besides, it would beso difficult to make separate use of Nb and Ta that a part of Taaddition might be substituted by Nb.

8 as a very small addition, can make'a remarkable improvement on thehigh temperature strength, but too much addition of B will result inheavy deterioration. Theroom temperature elongation, anti-oxidation,antisulphur and anti-Pb O characteristics. The appropriate addition of Bshould be 0.00010.0050 percent.

Y as a very small addition can improve the bonding of a protective filmat high temperature heating and vastly enhance the high temperatureanti-oxidation and anti-Pb characteristics. Whereas in Fe-Cr-Al alloyswhen they contain relatively low amounts of Cr and Al (Cr Al 3% theydevelop local and accelerated corrosion due to imperfectness of theprotective film of d-Al O which should bring about a high corrosionresistance at high temperatures, in the case of the invented alloy witha Y-content, this film can be compact and can adhere well to the basemetal, thereby compensating for the imperfectness of the film andexhibiting an excellent characteristic of high temperature corrosionresistance. Y is an expensive metal; its addition exceeding 1.5 percentwill not be so effective as might be expected and thus, 0.01-1.5 percentwill be the limits of its addition. Meanwhile, rare earth elements otherthan Y, i.e., Ce, La, Ca are practically as effective as Y andaccordingly, a partial or whole addition of Y may be substituted by rareearth elements such as Ce, La, Ca.

7 Ti is an elementwhich is added to the conventional Fe-Cr-Al alloys,too;-it is foundeffective through formation of TiC for, fixation ofsolid-solution carbon or prevention of crystal coarsening. Since toomuch Ti is likely to deteriorate the anti-oxidation characteristic andimpact strength of the alloy, it has been limited to 005-1 .0 percent.

Zr, just like Ti, is an element added to the conventional Fe-Cr-Alalloys, too. For the purpose of preventing the crystal coarsening itsaddition should be more than 0.01 percent, but it is an expensive metal;moreover, its excessive addition is likely to result in poor elongationat room temperature and lower impact strength. Thus, its appropriateaddition should be from 0.011.0 percent.

Be is also known as an element which as a verysmall addition can vastlyenhance the high temperature strength and can serve to prevent thecoarsening of crystals. Accordingly, it constitutes one of the essentialelements for the invented alloy, but it is extremely costly; besides,too much Be will deteriorate the room temperature elongation and theimpact strength. Thus, the optimum addition of it should be 0.01-2.0percent.

What is claimed is:

1. A heat-resistant, anti-corrosive alloy for high tem-' peratureservice, said alloy consisting essentially of:

less than 0.07 percent by weight carbon,

less than 0.5 percent by weight silicon,

15 to 21 percent by weight chromium;

2.5 to 4.0 percent by weight aluminum,

0.5 to 1.2 percent by weight molybdenum,

0.5 to 1.1 percent by weight vanadium,

less than 0.45 percent by weight titanium,

less than 0.18 percent by weight zirconium,

0.1 to 0.5 percent by weight of an element selected from thegroup'consisting of niobium and tantalum,

at least one element selected from the group consisting of: yttrium inan amount less than 1.5 percent by weight, beryllium in an amount lessthan 2.0 percent by weight, and boron in an amount less than 0.0050% byweight; and the balance of iron and inevitable impurities. 2. The alloyof claim 1, wherein yttrium is present in an amount in the range of0.03-0.4 percent by weight.

3. The alloy of claim 1, wherein beryllium is present in an amount inthe range of 00006-0001 percent by weight.

4. The alloy of claim 1, wherein at least one element selected from thegroup consisting of boron and beryllium is present in an amount lessthan 0.008 percent by weight.

1. A HEAT-RESISTANT, ANTI-CORROSIVE ALLOY FOR HIGH TEMPERATURE SERVICE, SAID ALLOY CONSISTING ESSENTIALLY OF: LESS THAN 0.07 PERCENT BY WEIGHT CARBON, LESS THAN 0.5 PERCENT BY WEIGH SILICON, 15 TO 21 PERCENT BY WEIGHT CHROMIUM; 2.5 TO 4.0 PERCENT BY WEIGHT ALUMINUM, 0.5 TO 1.2 PERCENT BY WEIGHT MOLYBDENUM, 0.5 TO 1.1 PERCENT BY WEIGHT VANADIUM, LESS THAN 0.45 PERCENT BY WEIGHT TITANIUM, LESS THAN 0.18 PERCENT BY WEIGHT ZIRCONIUM, 0.1 TO 0.5 PERCENT BY WEIGHT OF AN ELEMENT SELECTED FROM THE GROUP COCONSISTING OF NIOBIUM AND TANTALUM, AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OG: YTTRIUM IN AN AMOUNT LESS THAN 1.5 PERCENT BY WEIGHT, BERYLLIUM IN AN AMOUNT LESS THAN 2.0 PERCENT BY WEIGHT: AND BORON IN AN AMOUNT LESS THAN 0.0050% BY WEIGHT; AND THE BALANCE OF IRON AND INEVTIABLE IMPURITIES.
 2. The alloy of claim 1, wherein yttrium is present in an amount in the range of 0.03-0.4 percent by weight.
 3. The alloy of claim 1, wherein beryllium is present in an amount in the range of 0.0006-0.001 percent by weight.
 4. The alloy of claim 1, wherein at least one element selected from the group consisting of boron and beryllium is present in an amount less than 0.008 percent by weight. 